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Introduction to the P-Adic Space Introduction to the p-adic Space Joel P. Abraham October 25, 2017 1 Introduction From a young age, students learn fundamental operations with the real numbers: addition, subtraction, multiplication, division, etc. We intuitively understand that distance under the reals as the absolute value function, even though we never have had a fundamental introduction to set theory or number theory. As such, I found this topic to be extremely captivating, since it almost entirely reverses the intuitive notion of distance in the reals. Such a small modification to the definition of distance gives rise to a completely different set of numbers under which the typical axioms are false. Furthermore, this is particularly interesting since the p-adic system commonly appears in natural phenomena. Due to the versatility of this metric space, and its interesting uniqueness, I find the p-adic space a truly fascinating development in algebraic number theory, and thus chose to explore it further. 2 The p-adic Metric Space 2.1 p-adic Norm Definition 2.1. A metric space is a set with a distance function or metric, d(p, q) defined over the elements of the set and mapping them to a real number, satisfying: arXiv:1710.08835v1 [math.HO] 22 Oct 2017 (a) d(p, q) > 0 if p = q 6 (b) d(p,p)=0 (c) d(p, q)= d(q,p) (d) d(p, q) d(p, r)+ d(r, q) ≤ The real numbers clearly form a metric space with the absolute value function as the norm such that for p, q R ∈ d(p, q)= p q . | − | 1 We can modify the distance function to alter the notion of distance within a space, thus redefin- ing closeness. In the reals, two numbers are close together if their decimal representation is similar towards the right. For example, 3.1, 3.14, 3.141, 3.1415, ... are all growing closer to each other, while 15, 315, 1415, 31415, ... are all growing further apart. This is simply because appending a number n digits to the right of the decimal place only modifies its distance from another real number by 10−n, while appending a number n digits to the left of the decimal place alters its distance from another real by 10n. The p-adic numbers are quite the opposite. The p-adic space redefines the distance function, informally, such that two numbers are far if they are similar towards the right and close if they are similar towards the left. This is made rigorous through the p-adic norm. pnm Definition 2.2. We can represent any rational number n as n = n where p, m, and n are coprime. Z n Then, the p-adic valuation vp (n) = p . Formally, the p-adic valuation over the integers is a function vZ(n): Z N such that p → + n =0 Z v (n) := ∞ p max v : pv n, v N n =0 { | ∈ } 6 The p-adic valuation can be extended to the rationals as well, and the function vp(n): Q Z is → a Z Z defined as vp( )= v (a) v (b). b p − p Definition 2.3. The p-adic valuation is an auxillary function. It is used to clarify the definition of the p-adic norm, which is defined as p : Q N such that | · | → 0 n =0 n p := | | vp(n) p− n =0 6 By establishing the definition of the p-adic norm, we are now able to form a metric space over the p- adic numbers with the distance function defined as d(p, q)= p q p. We can now make some observations | − | about the nature of this space. The size of a p-adic number can be informally summarized as inversely proportional to the exponent of p in its prime factorization. It is clear that numbers that are larger in the real number space, per the absolute value function, tend to be smaller under the p-adic metric, since these numbers will be divisible by greater powers of p. Note that any number that is coprime to p will have size 1, since the valuation returns 0. 2.2 Completion of Q The set of rational numbers is the fundamental structure underlying much of number theory and math- ematics as a whole. 2 p Q = p, q Z, q =0 . q | ∈ 6 The rationals form a field under the addition and multiplication, satisfying the field axioms for each operation: commutativity, associativity, existence of an identity element, existence of an inverse element, and closure. However, upon examination we quickly discover inefficiencies in this field; for example, there does not exist a rational m such that m2 =2. An irrational number, √2 is one of the many gaps in the rational field, since we can get arbitrarily close to it from within the rationals but never reach it exactly. Closing these gaps under the real distance function yields the real numbers, while closing these gaps using the p-adic distance function yields the p-adic numbers. Before we can complete the rations, however, we must introduce some important definitions relating to elementary topology. Definition 2.4. For a metric space X, we define an open ball to be an open set B(x0, r) of all points x X centered around a point x with radius r such that d(x , x) < r. The corresponding closed ball is ∈ 0 0 said to be the closure of an open ball. Definition 2.5. A neighborhood of x0 is said to be an open ball Nr(x0) centered around a point x0 with radius r. Definition 2.6. A limit point of a set X is point p if every neighborhood of p overlaps with X, that is, for every neighborhood of p, there exists a point q = p such that q X Nr(p). 6 ∈ ∩ Definition 2.7. A Cauchy sequence is any sequence xi such that for every positive real number ǫ, there exists a natural number m such that d(xm, xm n) <ǫ for any n N. + ∈ Consider the sequence X = 1, 1, 6, 1.61, 1.614, ... in the real number space with each term including { } a subsequence digit of the decimal representation of √2. We can show that this is a Cauchy sequence by 1−i noting that all terms in the subsequence of X beginning with xi are within 10 of each other. Thus, for any ǫ, we can choose i such that 1 log ǫ i, which would give us a sequence xi, xi , ... such − ≤ { +1 } 1−i that for any n N, d(xi, xi n) 10 ǫ. A limit point of this Cauchy sequence is therefore √2 ∈ + ≤ ≤ since for any r > 0, we can choose an α > 0 and construct a neighborhood centered around √2 such that d(√2, √2 r α) = r α X. This is because of the fact for the nth term in the sequence is − − | − | ∈ 1−n within √2 xn 10 . In the rational space, all the limit points such as √2 that are formed under | − |≤ the absolute value function are contained in the set of irrational numbers. As such, we may construct the real numbers by completing the rationals, that is, including all the limit points of Q to yield R. This proof is quite complex and thus will not be covered in this paper, however, the general structure of the argument behind this proof is important since the p-adic numbers are constructed in the same fashion. While the real numbers represent the completion of Q under the the absolute value function which is the distance in the reals, the completion of Q induced by the p-adic metric forms the p-adic number set, 3 denoted by Qp. If we only consider integral numbers, we arrive at the set of p-adic integers, a subring of Qp denoted by Zp. The formal representation of a p-adic number is very similar to that of a p-ary number (a number in base p), as both are represented by a power series expansion of p. Definition 2.8. A p-adic number α can be represented, uniquely, for 0 ai <p as ≤ ∞ i α = aip Xi=n where the p-adic norm of α, α = p−n. | | 2.3 Arithmetic Operations One interesting consequence of the p-adic metric is that numbers no longer require a sign. That is, negative numbers are defined without the need for a negative sign. To illustrate this property, consider the 10-adic metric. In this space, considering a series of x 9s for increasing values of x, our number approaches 1. This is because − − − − 9 ( 1) = 9 = 10 1, 99 ( 1) = 102 = 10 2, ... 10x 1 ( 1) = 10x = 10 x | − − |10 | | | − − |10 | | | − − − | | | −x lim 10 10 =0 x→∞ | | . Thus, the distance between 999...9 and 1 is considered to be 0. Since this technique involves − constantly appending to the left, in the 10-adic system, we can represent this same process involving x limx→∞ 10 1 as ...999 = 1. | − |10 − This ring of 10-adic (decadic) numbers is very useful to demonstrate the relationship between the real numbers and the p-adic numbers. Let us now explore the function of transformations via the fundamental operations in 10-adic space. We can define the basic operations in the 10-adic space: addition and multiplication, thus they form a commutative ring (note that while p-adic spaces form fields for some values of p, all p-adic spaces form rings). Addition and multiplication with p-adic numbers function the same as with real numbers, the only difference being that p-adic numbers can extend infinitely to the left.
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